The present invention relates to a sodium-sulfur storage battery, and more particularly, to a structure thereof.
Generally, a sodium-sulfur storage battery is a secondary battery of a high temperature type, in which sodium used as negative reactant and sulfur used as positive reactant are completely separated from each other by a sodium ion conductive solid electrolyte tube such as beta-two-dash alumina, and is a high performance battery having theoretical capacities of which discharging capacity is 100 with respect to a charging capacity of 100, i.e., a ration of discharging and charging capacities is 100:100.
However, in actual use, said theoretical capacities can not be obtained because of problems such as low utilizing factor of the negative reactant, low utilizing factor of the positive reactant and insufficient gastightness in a thermocompressively jointed portion.
The above problem is detailed below with reference to FIG. 2, which is a sectional view of a conventional sodium-sulfur storage battery. In FIG. 2, an upper end of a solid electrolyte tube 1 is jointed with solder glass to an alpha-alumina ring 2, of which lower surface is thermocompressively jointed to a positive cover 3 with an aluminum layer therebetween. The outer periphery of the cover 3 is welded to an upper end of a battery housing 4. Sulfur used as positive reactant 5 is impregnated in positive electroconductive material 6 such as graphite felt and is disposed between the battery housing 4 and the solid electrolyte tube 1. A bottom cover 7 is arranged in the housing 4 and is welded at its periphery to the lower end of the housing 4. Metallic fiber 9 such as stainless steel is filled in the solid electrolyte tube 1. A negative collector tube 10 welded to a negative cover 11 is inserted into the tube 1. The outer periphery of the negative cover 11 is welded to a negative auxiliary cover 8 which is thermocompressively jointed to the upper surface of the alpha-alumina ring 2 with an aluminum layer therebetween. The negative collector tube 10 is sealed by vacuum welding after sodium used as negative reactant 12 is impregnated in vacuum condition through the negative collector tube 10.
The sodium-sulfur storage battery having above-noted structure has following disadvantages. The sodium used as the negative reactant 12 spreads above the metallic fiber 9 filled in the solid electrolyte tube 1, and said spread sodium does not contribute to the discharging, which reduces the utilizing factor of the negative reactant. Further, since the above spread sodium occupies a position higher than an upper surface 6' of the positive electroconductive material 6, it may contact the positive reactant 5 through the solid electrolyte tube 1 and react directly with it, in which case heat is generated. By this heat, the solid electrolyte tube 1 may be broken, and the thermocompressively jointed portion of the alpha-alumina ring 2 and the negative auxiliary cover 8 may be corroded, which causes insufficient gas tightness. On the other hand, when the positive reactant 5 discharges, the volume thereof increases and the liquid level of the positive reactant 5 rises, so that the reactant 5 wets the upper inner surface of the battery housing 4 and the lower surface of the positive cover 3. In the subsequent charging operation, the reactant 5 which has wetted the surfaces may remain there without being charged. This reduces the utilizing factor of the positive reactant 5. Further, the positive reactant 5 which has wetted the lower surface of the positive cover 3 may cause corrosion and insufficient gastightness at the thermocompressively jointed portion of the positive cover 3 and the alpha-alumina ring 2.
Accordingly, it is an object of the invention to provide a sodium-sulfur storage battery, in which utilizing factors of positive and negative reactant are improved, and in which insufficient gastightness and breakage of a solid electrolyte tube are prevented.